In lithographic printing, a printing plate is mounted on the cylinder of a printing press. The printing plate carries a lithographic image on its surface and a printed copy is obtained by applying ink to the image and then transferring the ink from the printing plate onto a receiver material, which typically is a sheet of paper. Generally, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the surface of the receiver material (offset printing).
In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. When the surface of the printing plate is moistened with water and ink is applied, the hydrophilic regions retain water and repel ink, and the ink-receptive regions accept ink and repel water. During printing, the ink is transferred to the surface of the receiver material upon which the image is to be reproduced.
Lithographic printing plates typically comprise an imageable layer (also called imaging layer or coating) applied over the hydrophilic surface of a substrate, typically aluminium. The imageable layer includes one or more radiation-sensitive components, often dispersed in a suitable binder.
To produce the lithographic image on the printing plate, the printing plate is imaged by targeted radiation. This can be carried out in different ways. In direct digital imaging (computer-to-plate), printing plates can be imaged with infrared or UV lasers or light sources. Such a laser beam can be digitally controlled via a computer; i.e. the laser can be turned on or off so that imagewise exposure of the precursor can be affected via stored digitized information in the computer. Therefore, the imageable layers of printing plates, which are to be imagewise exposed by means of such image-setters, need to be sensitive to radiation in the near-infrared (NIR) or ultraviolet (UV) regions of the spectrum.
The imaging device will etch the image on the printing plate by eliciting a localized transformation of the imageable layer. Indeed, in such imaged systems, the imageable layer typically contains a dye or pigment that absorbs the incident radiation and the absorbed energy initiates the reaction producing the image. Exposure to radiation triggers a physical or chemical process in the imageable layer so that the imaged areas become different from the non-imaged areas and development will produce an image on the printing plate. The change in the imageable layer can be a change of hydrophilicity/oleophilicity, solubility, hardness, etc.
Following exposure, either the exposed regions or the unexposed regions of the imageable layer are removed by a suitable developer, revealing the underlying hydrophilic surface of the substrate. Developers are typically aqueous alkaline solutions, which may also contain organic solvents.
Alternatively, “on-press developable” lithographic printing plate can be directly mounted on a press after imaging, and are developed through contact with ink and/or fountain solution during initial press operation. In other words, either the exposed regions or the unexposed regions of the imageable layer are removed by the ink and/or fountain solution, not by a developer. More specifically, a so-called on-press development system is one in which an exposed printing plate is fixed on the plate cylinder of a printing press, and a fountain solution and ink are fed thereto while revolving the cylinder to remove non-image areas. This technique allows an imaged, but un-developed printing plate (also called a printing plate precursor) to be mounted as is on a press and be made into a printing plate on an ordinary printing line.
If the exposed regions are removed, the precursor is positive working. Conversely, if the unexposed regions are removed, the precursor is negative working. In each instance, the regions of the imageable layer (i.e., the image areas) that remain are ink-receptive, and the regions of the hydrophilic surface revealed by the developing process accept water and aqueous solutions, typically a fountain solution, and do not accept ink.
Negative-working lithographic printing plates using a variety of polymers, copolymers, polymeric particles and binders have already been developed. However, there remains a need for new materials and new coatings for such plates.